International Publication Number WO 2007/012181, Apparatus and Method for Treating FGD Blowdown or Similar Liquids, describes a fixed film reactor having a granular activated carbon (GAC) bed. The reactor can be used to treat water containing one or more of nitrate, selenium and sulfates. In one example, feed water flows downwards through the bed. Nitrate is removed in an upper part of the bed by biological conversion to nitrogen gas. Selenium is removed in a lower part of the bed by biological reduction of soluble selenium ions (i.e. selenate and selenite) into particles of elemental selenium. Nitrate is removed first because nitrate is a preferred electron acceptor for biological reduction over selenate or selenite. A suitable reactor is sold by GE Water & Process Technologies under the ABMET trade mark.
Reactors as described above have a grid of porous pipes below the GAC bed and an array of overflow troughs above the GAC bed. Precipitates (including excess biomass and selenium particles) are removed by periodically backwashing the bed with water fed through the under bed pipes and withdrawn by way of the overflow troughs. The backwashing water fluidizes the bed, which expands into a space provided between the top of the bed and the overflow troughs.
Nitrogen gas bubbles also accumulate in the bed. These bubbles are removed by briefly flowing water through the under bed pipes to expand the bed. Gas bubbles are released as the bed expands, and float upwards. In WO 2007/012181, this process is called “bumping” and happens more frequently than backwashing, but requires less time.